Natural oils and fats are mainly composed of triglycerides. Triglycerides are triesters of fatty acids and glycerol. Vegetable oils and animal fats can also contain some free fatty acids (FFA), which are formed during production of oils and fats through hydrolysis of triglyceride. These raw materials (in the following also referred to as “glyceride raw material”) can be used as a valuable feedstock for the production of diesel grade fuels.
Approaches for converting vegetable oils or other fatty acid derivatives into liquid fuels comprise transesterification, catalytic hydrotreatment, hydrocracking, catalytic cracking without hydrogen and thermal cracking, among others. In the hydrotreatment of vegetable oils (HVO) and animal fats, hydrogen is used to remove oxygen from the triglyceride vegetable oil molecules and to split the triglyceride into separate chains thus creating hydrocarbons. During hydrotreatment, for example, hydrodeoxygenation, oxygen containing groups are reacted with hydrogen and removed through formation of water and therefore this reaction employs rather high amounts of hydrogen. Decarboxylation and decarbonylation can occur as side reactions in hydrodeoxygenation producing carbon dioxide and carbon monoxide, respectively.
A catalytic method for the manufacture of hydrocarbons, which are suitable for diesel fuel, from renewable sources, such as plant and vegetable oils and fats and animal and fish oils and fats, is disclosed in EP 1 681 337. The process includes the step of transforming the starting materials into hydrocarbons with minimal consumption of hydrogen, by contacting the starting material with a heterogeneous catalyst comprising at least one metal selected from the metals belonging to the group VIII of the Periodic Table. The hydrocarbons formed via decarboxylation/decarbonylation reactions have one carbon atom less than the original fatty acid or fatty acid portion of its derivate.
The products obtained by the above-mentioned process have a chemical composition which corresponds to that of traditional diesel. They can be blended with fossil diesel, traditional biodiesel (FAME), or used as such in diesel engines.
During the conversion of a glyceride raw material by deoxygenation reactions, off-gases are formed which, depending on the raw material and the reaction conditions, contain various concentrations of impurities which impair processing of the raw material and potentially even the product properties of the final products. Impurities can include sulphide compounds, such as H2S and COS, ammonia, and halogenides, such as chloride compounds. The latter compounds are primarily formed during processing of the feed. For example, nitrogen compounds give ammonia, and chlorides give hydrochloric acid at the conditions employed in a hydrodeoxygenation reactor. The concentration of ammonia and chlorides are at the ppm level in gas volumes withdrawn from deoxygenation reactions.
Regarding the sulphides, there are various sources. In a hydrodeoxygenation reactor, the catalyst metals can be active in sulphided form and the presence of sulphur or sulphurous compounds during the operation of the reactor can be required for maintaining catalyst activity. For this purpose, some sulphur compounds are actively recycled from a point downstream of the process. Another source of sulphur compounds or sulphides is represented by the feed which usually contains minor amounts of sulphide compounds. However, in practice, there can still be a need for the introduction of fresh (external) sulphur compounds for process control.
As a result of the cumulative sulphur sources, the effluent gases of the hydrodeoxygenation reactor will contain sulphide compounds, for example, in concentrations of 10 to 2000 vol-ppm.
Hydrodeoxygenation can be carried out using excess hydrogen. Then, unreacted hydrogen is recovered and recycled. Hydrogen rich off-gas can be subjected to amine wash using, for example, monoethanolamine (MEA) or diethanolamine (DEA) to remove carbon dioxide. The amine will also remove sulphide compounds which will contaminate the amine and separation of carbon dioxide from the sulphide compounds from the amine compounds can require special arrangement.
For removing impurities, such as sulphides, from carbon dioxide rich gas at least one, a plurality of separate treatment steps are employed. Thus, various absorption materials can be utilized for separating sulphur compounds and carbon dioxide. These separation and wash processes are commercially applied for effective CO2 removal at low sulphur contents. In addition, various heat treatments or guard beds are used.
WO 98/55209 discloses a method and system for removal of sulphur and sulphur-containing compounds from gas flows using aqueous metal salt solutions at acid conditions. The metal is regenerated by treating the sulphide precipitation at higher temperature with hydrogen to yield pure metal and H2S. The H2S can be further treated for instance in a Claus unit to give elementary sulphur. WO 98/55209 is directed to the treatment of gas flows obtained of natural gas, coal gas or biogas, and similar hydrocarbon sources, employing high concentrations of the metal salts.